1. Field of the Invention
This invention relates to stringed musical instruments and more particularly to an improved bracing structure for the soundboards of such instruments.
2. The Prior Art
In stringed musical instruments such as guitars, violins, pianos, and the like, a plurality of tightly stretched strings pass over a bridge structure, the bridge structure physically engaging the instrument soundboard. The soundboard forms one side of an airfilled cavity. When one or more strings are caused to vibrate by being struck, bowed, or otherwise, the frequency at which the string vibrates is determined by the material and length of the string, its tension, and by its caliper (thickness or weight.) In addition to vibrating at the fundamental frequency of the string determined by the factors indicated above, the string also vibrates at certain higher harmonics of this frequency, the extent of these higher harmonics depending upon a number of factors including the caliper and tension of the string. Such string vibration alone is largely inaudible, primarily because of air circulation around the vibrating string.
The bridge mechanically couples the vibration of the string or strings to the instrument soundboard causing vibration of the soundboard which is a function both of the vibrating frequencies of the strings(s) and of the physical characteristics of the soundboard. Ideally, the soundboard should vibrate with substantially uniform amplitude over the full frequency range of the instrument, providing a substantially uniform, accurate response. However, since the soundboard is essentially a vibrating plate, it has its own limitation on natural vibrating frequencies and careful design is therefore required in order to achieve the desired frequency response. Vibrations of the soundboard are coupled to the resonant cavity of the instrument causing vibrations therein, which likewise are a function both of the coupled vibrations of the soundboard and of the resonant characteristics of the cavity. The sound output from the instrument is determined primarily by the vibrations of the soundboard and the cavity.
Thus, it is seen that both the tonal characteristics and the strength or intensity of the sound obtained from a stringed musical instrument are dependent to a large extent on the characteristics of the instrument soundboard. While the teachings of this invention are suitable for application with the soundboards of most stringed musical instruments, for purposes of illustration, the discussion to follow will be concerned primarily with the soundboards of accoustical guitars. An acoustic guitar has a number of strings, such as, for example, six strings, with higher frequency or treble strings on one side and lower frequency or bass strings on the other side, the strings vibrating at successively lower frequency as one moves from the treble side to the bass side. These strings, particularly in steel string models, are under substantial tension. Because of the manner in which the strings are mounted, the strings apply static rotational or torsional forces to the bridge. The forces applied to the bridge also vary dynamically as the string is vibrated. Since the bridge is attached to the soundboard, the torsional forces applied to the bridge are transmitted through the bridge to the area of the soundboard directly thereunder. These relatively large forces applied to a relatively small area can cause bending, cracking, or other damage to the soundboard and are one of the principal causes of the soundboard failure.
In order to protect the soundboard against damage due to torsional forces, attempts have been made to strengthen or stiffen the soundboard by securing bracing members to the under side of the soundboard and by other means. However, in addition to transmitting torsional forces to the soundboard, the bridge also transmits the forces resulting from string vibration to the soundboard. In order to achieve full, brilliant tones from the guitar, the soundboard must be able to vibrate strongly in response to these vibrational forces. However, an overly stiff soundboard tends to damp these vibrational forces, resulting in a dull, weak, clearly undesirable sound output. Thus, a conflict has for some time existed between the mechanical and the sonic requirements of the soundboard.
Fortunately, the undesired torsional forces applied to the soundboard are in a direction parallel to the plane of the soundboard whereas the desired vibrational forces are in a direction perpendicular to the plane of the soundboard. Therefore, it is possible, in accordance with the teachings of this invention, to provide a soundboard having the required strength against torsional forces while still being adapted to respond strongly to vibrational forces, providing a strong, brilliant output.
Since the soundboard is essentially a vibrating plate, its geometry causes it to have certain natural resonant characteristics. The soundboard thus responds more strongly to certain applied vibrational frequencies, and less strongly to other frequencies. Heretofore, no systematic effort has been made to provide an instrument adapted to respond uniformly over the full frequency range of the instrument, or alternatively, to respond in accordance with a predetermined response characteristic. Instead, the response characteristics of a particular instrument have been more or less empirically selected. In particular, attempts have been made to control the acoustic response of the soundboard through the use of bracing bar patterns attached to the underside of the soundboard. The purpose of these patterns is to define certain zones of the soundboard in which it will be permitted to vibrate at certain frequencies, and other zones in which the soundboard is permitted to vibrate at other frequencies. However, because of the manner in which these bars have been placed, they frequently served to define nodes rather than perimeters, with antinode vibrations occurring in regions of the soundboard outside of the desired zones. These antinode vibrations cause energy losses which weaken the output from the instrument and muddy the outputs obtained therefrom.
From the above, it is clear that a systematic approach to the design of the soundboard, and in particular to the design of the bracing pattern for the soundboard, of stringed musical instruments in general, and guitars in particular, is required. Such a systematic design should provide the required structural strength to permit the soundboard to withstand the torsional forces applied thereto while not interfering with the sonic or acoustical vibrations of the instrument and should permit strong vibration of the instrument at each desired frequency in a selected zone thereof, without permitting antinode vibrations to occur outside the selected zones.
In accordance with the above, this invention provides a bracing structure formed on the underside of the soundboard of a stringed musical instrument having lower frequency or bass strings and higher frequency or treble strings, the soundboard being coupled to these strings through a bridge structure. The bracing structure includes a torsion bar positioned substantially in axial alignment with the bridge, torsional forces applied by the strings to the bridge being transmitted to the torsion bar and through the torsion bar to the soundboard and the frame of the instrument. Framing means are provided which are physically connected to and act through the torsion bar to distribute the torsional forces from the bridge area of the soundboard. For preferred embodiments, means are provided for structurally supporting the soundboard and the framing means includes means for distributing the torsional forces to the means for structurally supporting. For preferred embodiments, the framing means also includes a plurality of framing bars, each of which is attached at one end to the torsion bar and extends along the underside of the soundboard in a predetermined direction away from the torsion bar, each of the framing bars being attached to the underside of the soundboard and serving to transmit the forces applied to the bridge area of the soundboard over a larger area thereof.
The bracing structure also includes a plurality of acoustical bars attached to the underside of the soundboard and oriented in a predetermined pattern, said bar pattern being adapted to permit the soundboard to vibrate optimally at different frequencies in different zones thereof, and boundary means for limiting the zones of the soundboard which may vibrate at selected frequency ranges, and for inhibiting the formation of antinodes outside of these limited zones. For preferred embodiments, the boundary means includes stiffening means attached to the underside of the soundboard in the regions thereof outside of the limited zones which may vibrate at the selected frequency ranges. More specifically, the boundary means includes at least one peripheral bar attached to the underside of the soundboard and positioned substantially at the junction between a limited zone and a region outside the limited zone, and the stiffening means are stiffening bars attached to the underside of the soundboard in the outside region.